1. Field of the Invention
The present invention relates to a method for producing a thermoplastic resin composition. Particularly, it concerns a method for producing, with industrial advantages, a thermoplastic resin composition having excellent impact resistance and fewer volatile components by mixing thermoplastic resins whose melting temperatures or glass transition points differ by 10.degree. C. or more with respect to each other.
2. Description of the Related Art
Thermoplastic Resin (A) having poor flowability and impact strength in spite of good heat resistance has been used by mixing it with a thermoplastic resin and/or an elastomer and the like which has a lower melting temperature or glass transition point than Thermoplastic Resin (A) by 10.degree. C. or more. Conventional methods for mixing such resins and/or an elastomer and the like having a melting temperature or glass transition point which is at least 10.degree. C. lower than that of Thermoplastic Resin (A) are exemplified as follows:
(1) a method comprising mixing a powder or pellets of both resins in the form of a solid using a tumbler, melt mixing the resultant mixture with a kneader such as a single-stage melt extruder to pelletize it, and injection or extrusion molding the obtained pellets to produce a molded article; PA1 (2) a method comprising mixing a powder or pellets of both resins in the form of a solid using a tumbler and subsequently injection or extrusion molding the resultant mixture to produce a molded article; and PA1 (3) a method comprising mixing Thermoplastic Resin (A) with a small amount of Thermoplastic Resin (B), which has a melting temperature or glass transition point which is at least 10.degree. C. lower than that of Thermoplastic Resin (A), in the form of a solid, pelletizing the resultant mixture with a kneader such as a melt extruder, further mixing the resultant pellets with Thermoplastic Resin (C) and the like, which also has a melting temperature or glass transition point which is at least 10.degree. C. lower than that of Thermoplastic Resin (A), in the form of a solid, and pelletizing the mixture with a kneader such as a second melt extruder (Japanese Patent Application Laid-Open No. 117444/1992).
However, according to Method (1), at the step of kneading with a melt extruder, which is carried out for mixing the resin uniformly, the resin temperature disadvantageously increases in order to melt Component A so that Component B and the like deteriorate. Accordingly, the impact strength and like properties of the resultant resin composition become poor. If the resin temperature is lowered to avoid the deterioration of impact strength and the like, the resins cannot be well kneaded and also the amount of the volatile component cannot be reduced. If the amount of the elastomer to be added is increased, the amount of the volatile component cannot be reduced though the impact resistance is improved.
According to Method (2), silver streaks and the like appear on the resultant molded article because Component A is not deaerated (i.e., volatile components of Component A are not removed). When Component A is not deaerated (i.e., volatile components of Component A are not removed), Thermoplastic Resin (A) is not melt mixed sufficiently since Thermoplastic Resin (B) melts prior to the melting of Thermoplastic Resin (A) and particles of non-melted Thermoplastic Resin (A) are produced. Further, a sufficient impact strength cannot be achieved.
In the above Method (3) (a master batch method), the melt mixing is carried out in two steps. Therefore, the resin is deteriorated so that the impact strength is reduced. In addition, this method has drawbacks such as disadvantageous energy consumption and a complicated operation.
Japanese Patent Application Laid-Open No. 149917/1995 discloses a method for producing a thermoplastic resin composition wherein, when 100 parts by weight of a composition comprising a polyphenylene ether and an aromatic alkenyl compound polymer is melt mixed in the presence of one or more parts by weight of an organic solvent using an extruder, polyphenylene ether, a part of an aromatic alkenyl compound polymer and an organic solvent are fed from a raw material supply port located at the upper point of the resin stream and the residual part of the aromatic alkenyl compound polymer is fed from a raw material supply port located at the lower point of the resin stream. According to this method, the organic solvent is kneaded with a mixture of polyphenylene ether and the aromatic alkenyl compound polymer in an extruder so that the components are not sufficiently mixed. When the amount of the organic solvent is increased for the purpose of sufficient mixing, removal of the organic solvent becomes difficult and further the extrusion productivity falls and the extrusion amount of the resin changes due to back flow of the gas towards the first raw material supply port.